Cap for a writing instrument



FIG. 1 is a perspective view of a cap for a writing instrument showing my new design;

FIG. 2 is a front elevation view thereof;

FIG. 3 is a bottom plan view thereof, the top plan view being a mirror image thereof;

FIG. 4 is a rear elevation view thereof;

FIG. 5 is a right side elevation view thereof; and,

FIG. 6 is a left side elevation view thereof. 

The ornamental design for a cap for a writing instrument, as shown and described. 